Although there are many different configurations of post drivers, they all have in common the presence of a hammer, and some form of guide means for directing the hammer from a raised position along a substantially vertical downward path to impact upon a post. Some means must be provided for lifting the hammer to the raised position and then releasing the hammer to begin its downward path. Sometimes the hammer falls by force of gravity alone and sometimes the fall of the hammer is accelerated by means of springs.
Hydraulic and pneumatic post drivers use fluid expandable cylinders to lift the hammer of the post driver to the raised position. Hydraulic post drivers are described and illustrated in U.S. Pat. No. 2,940,267 and U.S. Pat. No. 4,915,180. A limiting factor for these post drivers is their ability to release the hammers. As long as the hydraulic or pneumatic fluid lines remain attached to the hammer, they unavoidably have the effect of dampening the potential impact; for it is the fall of the hammer that vacate the fluid lines.